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Author Topic: Simple 2-way radio mic antenna solution  (Read 686 times)

Chris Shaw

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Simple 2-way radio mic antenna solution
« on: September 08, 2019, 04:00:39 am »

Good morning,

I'm planning to purchase a couple of Sennheiser EW100 radio mics and need a simple solution for combining and elevating their antennae that avoids unnecessary cost. It is a portable system and we will be operating in the 863 to 865 MHz band.

They come with 1/4 wave antennae and the manual instructs to use the Sennheiser active antenna combiners which are both expensive and take up half a rack space. Hence I need another solution.

So I have some questions:

1) What is the difference between e.g. a Sennheiser A1031-U antenna and a (correct length) 1/2 wave stick antenna? (apart from the mounting thread and price tag)

2) What is the difference between e.g. a Shure UA221 passive antenna splitter and a 50 ohm BNC T-piece? (apart from some black plastic and the price tag?)

3) Would using 1/2 wave sticks with BNC T-pieces as above be unadvisable? (it would save me budget to do a better job of packaging)

4) Does anyone know of any off-the-shelf bracket for mounting a mic stand boom arm or similar pole to a rack? Ideally I will manage to store the whole lot in the 4U front of house rack.

Thanks very much
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Jean-Pierre Coetzee

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Re: Simple 2-way radio mic antenna solution
« Reply #1 on: September 08, 2019, 05:39:02 am »

This has come up a gazillion times already but will rehash it again. RF is brand agnostic so a correct length 1/2 wave from sure will work just fine on Sennheiser etc.

The difference between the passive splitter and a T-piece is quite significant. When splitting RF signal you want to still present the antenna and the receiver with the correct impedance(which is 50 Ohms BTW) and the passive splitter has circuitry that ensures that, sure you do not need to buy the Shure passive splitter, I'm pretty sure that Sennheiser also has an option for that, RFVenue has also had an option in the past and you can also search the forums for that topic and there is a way to get the OEM part in-stead of buying from one of the big brands, they pretty much all use the same OEM part. I don't have the model number on hand but really just search the forum its in a lot of  threads.

When buying cable for longer runs don't use RG58/59(one is 75 Ohm and the other is 50) , you want to use LMR 400 or better, once again I'm just pointing you in the right direction, search the forums its been discussed to death.

So to summarise:

1.) None whatsoever, as long as the antenna is in the correct frequency range you can use it.
2.) Massive, use  a passive splitter
3.) People generally fabricate something themselves but there was a thread discussing literally this only a month or two ago, go and look for it.

Extra:

Use LMR400 cable for longer runs, pretty much anything better than RG58/59 is going to be better for you .
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Keith Broughton

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Re: Simple 2-way radio mic antenna solution
« Reply #2 on: September 08, 2019, 06:23:28 am »

JP covered it pretty well but I would like to add a couple of points.
To repeat...do not use the T adapter! Someone here might suggest a product from Mini Circuits for passive splitting that may be less expensive.
Also, do not use the 1/4 wave antennas supplied with the kit. Use 1/2 wave or "paddles" when mounting on a stand.
By the sound of it, your antennas will be close to the receivers so LMR 400 cable won't be too expensive for a short run.
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Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #3 on: September 13, 2019, 03:15:47 pm »

May I ask, what's the difference between this guy https://en-us.sennheiser.com/asp-212 and a T-adapter?

Just don't use any of them as combiners, but can do as splitters, or?
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Jean-Pierre Coetzee

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Re: Simple 2-way radio mic antenna solution
« Reply #4 on: September 13, 2019, 03:25:08 pm »

Far as I know you can use that guy as a splitter or combiner but I might be wrong. There is circuity in that box that matches the impedance of the outputs and the input. A T-Piece with cause the impedance to half at the output which will cause you to lose half your signal due to the signal being reflected back on itself.

At RF frequencies a major change in impedance acts quite a lot like a wall, you want to avoid that as much as possible.

This is probably the mini-circuits unit that I referenced before but couldn't remember the model number for. Will do what you want but will be cheaper than the sennheiser/shure/rfvenue options
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Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #5 on: September 13, 2019, 03:29:48 pm »

Far as I know you can use that guy as a splitter or combiner but I might be wrong. There is circuity in that box that matches the impedance of the outputs and the input. A T-Piece with cause the impedance to half at the output which will cause you to lose half your signal due to the signal being reflected back on itself.

At RF frequencies a major change in impedance acts quite a lot like a wall, you want to avoid that as much as possible.

This is probably the mini-circuits unit that I referenced before but couldn't remember the model number for. Will do what you want but will be cheaper than the sennheiser/shure/rfvenue options

The link you provided is bad, even after removing what's before https.

But thanks. I believe we've used them as combiners before, not sure though, just had to ask.
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Jean-Pierre Coetzee

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Re: Simple 2-way radio mic antenna solution
« Reply #6 on: September 13, 2019, 03:34:20 pm »

I corrected it almost immediately. When I clicked on it I noticed. Should be right now.
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Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #7 on: September 13, 2019, 03:41:20 pm »

I corrected it almost immediately. When I clicked on it I noticed. Should be right now.

There we go.

Maybe this should be singled out into it's own thread, a mod can make the call and take action if this does not belong in this thread.

The ASP 212 and the one you provided are transformers. I get it now, a simple T is what it it, a simple T.

After reading on of the papers in the link you provided: Let's say you use it as a combiner. Is there any chance of 180 phase issue, or close to it, when using different cable lengths/gauges?
« Last Edit: September 13, 2019, 03:45:54 pm by Miguel Dahl »
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Jean-Pierre Coetzee

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Re: Simple 2-way radio mic antenna solution
« Reply #8 on: September 13, 2019, 04:04:59 pm »

My best bet would be to try to get Henry or Pete or Jason to respond to that one, I'm really not sure since I've never really used one as a combiner.
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Jason Glass

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Re: Simple 2-way radio mic antenna solution
« Reply #9 on: September 13, 2019, 10:01:29 pm »

After reading on of the papers in the link you provided: Let's say you use it as a combiner. Is there any chance of 180 phase issue, or close to it, when using different cable lengths/gauges?

Phase shifting of TX combiner inputs has no detrimental affect on transmission quality when combining IEM carriers that operate at different frequencies.

However, it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications, but is not something we should ever intentionally do when combining IEM carriers for TX.
« Last Edit: September 13, 2019, 10:03:52 pm by Jason Glass »
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Henry Cohen

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Re: Simple 2-way radio mic antenna solution
« Reply #10 on: September 13, 2019, 10:31:00 pm »

Phase shifting of TX combiner inputs has no detrimental affect on transmission quality when combining IEM carriers that operate at different frequencies.

However, it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications . . .

And in high power broadcast applications. There is no such thing as a 500kW amplifier power transistor: Multi 10kW and 100kW power levels are achieved by coherently combining dozens or scores of 100+W transistors to create 2.5kW and 5kW amplifier pallets, which are in turn coherently combined to achieve 100's of kW, all with phase accuracies measured in hundredths of degrees.
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Daniel Levi

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Re: Simple 2-way radio mic antenna solution
« Reply #11 on: September 14, 2019, 03:46:49 am »

And in high power broadcast applications. There is no such thing as a 500kW amplifier power transistor: Multi 10kW and 100kW power levels are achieved by coherently combining dozens or scores of 100+W transistors to create 2.5kW and 5kW amplifier pallets, which are in turn coherently combined to achieve 100's of kW, all with phase accuracies measured in hundredths of degrees.

Or you use Klystrons/Klystrodes, of course, but that's a bit more "pro gamer  ;) " and not really relevant to the thread, plus if you need that much power for your IEM transmitter then you need a different forum  ;D.

And as you may well know Klystrons/Klystrodes are still very much used in modern broadcast applications, despite the advent of high power transistor based transmitters. The Transmitter at Rowridge on the Isle-of-Wight (http://tx.mb21.co.uk/gallery/gallerypage.php?txid=560 https://web.archive.org/web/20170310145119/http://www.theonlineengineer.org/TheOLEBLOG/rowridge-transmission-site/) uses Thomson transmitters that were new in ~2010 and use Klystrodes for the output stage. 
« Last Edit: September 14, 2019, 03:49:32 am by Daniel Levi »
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Jason Glass

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Re: Simple 2-way radio mic antenna solution
« Reply #12 on: September 14, 2019, 11:06:51 am »

Or you use Klystrons/Klystrodes, of course, but that's a bit more "pro gamer  ;) " and not really relevant to the thread, plus if you need that much power for your IEM transmitter then you need a different forum  ;D.

Tubes, baby!  Not just for guitar amps and microwave ovens.   ;)

Since we're getting into it, intentional phase shift makes antenna arrays possible, and is critical to beam steering applications such as radar.

https://en.m.wikipedia.org/wiki/Phased_array

Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #13 on: September 15, 2019, 04:31:34 pm »

Phase shifting of TX combiner inputs has no detrimental affect on transmission quality when combining IEM carriers that operate at different frequencies.

However, it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications, but is not something we should ever intentionally do when combining IEM carriers for TX.

How about combining two RX antennas?
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Jason Glass

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Re: Simple 2-way radio mic antenna solution
« Reply #14 on: September 15, 2019, 04:41:50 pm »

How about combining two RX antennas?

That's what DAS is.  Distributed Antenna System.

...it has enormous affect when combining carriers at the same frequency.  This is common in various DAS, laboratory, measurement, and circuit design applications...
« Last Edit: September 15, 2019, 04:48:12 pm by Jason Glass »
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Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #15 on: September 16, 2019, 11:55:08 am »

That's what DAS is.  Distributed Antenna System.

Either I don't get something here, or I wasn't clear.

Let's say you have two zones you want to pick up some handhelds or whatever from. Wouldn't a combiner be the correct unit to use?

For instance, real life application here: An outdoor theater, two antennas covering the main "stage" as usual, but one also want to cover an area which is "in the RF-shadows" of the two main antennas. Could one use the ASP-212 to add a third antenna? The ASP-212 is afaik a splitter/combiner. Basically having two antennas going through one coax using a "dumb" combiner, is my point I guess.

I saw RF venue has a unit called 4 Zone. This thing does this as far as I understood. But could one use the ASP 212 to combine "out in the field", and not at WL-land?
« Last Edit: September 16, 2019, 11:57:14 am by Miguel Dahl »
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Jason Glass

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Re: Simple 2-way radio mic antenna solution
« Reply #16 on: September 16, 2019, 12:50:04 pm »

Either I don't get something here, or I wasn't clear.

Let's say you have two zones you want to pick up some handhelds or whatever from. Wouldn't a combiner be the correct unit to use?

For instance, real life application here: An outdoor theater, two antennas covering the main "stage" as usual, but one also want to cover an area which is "in the RF-shadows" of the two main antennas. Could one use the ASP-212 to add a third antenna? The ASP-212 is afaik a splitter/combiner. Basically having two antennas going through one coax using a "dumb" combiner, is my point I guess.

I saw RF venue has a unit called 4 Zone. This thing does this as far as I understood. But could one use the ASP 212 to combine "out in the field", and not at WL-land?

Yes, you are describing a distributed antenna system, and correctly understanding how to use a combiner in that application.

Shure shows how to engineer this scenario here https://www.shure.com/en-US/support/find-an-answer/antenna-distribution-2-rooms-4-antennas

Of the many factors to consider when designing such a system, the most important is to avoid conditions where a single carrier, transmitted from a desired performance location, arrives at the receiver through multiple paths that sum to a RSS that is low enough to cause a dropout or audible noise.  Since microphone TX are mobile devices and not rigidly fixed in spatial relation to their RX antennas, each path will impart a unique and dynamic loss or gain and phase shift on the signal.  Combiners and splitters are components that can cause phase shift, as are amplifiers and the electrical length of the entire path.

One of the easiest and most inexpensive ways to mitigate a destructive summing of zones is to insert variable attenuation or amplification into each path, which the user can adjust during or after system deployment.  Variable phase shifting would be wonderful, but I suspect way too expensive.  It's also worth noting that receiver diversity is an enormous factor in DAS, since by design it makes momentary phase cancellations on one side of the diversity pair inaudible.
« Last Edit: September 16, 2019, 01:12:05 pm by Jason Glass »
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Miguel Dahl

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Re: Simple 2-way radio mic antenna solution
« Reply #17 on: September 16, 2019, 01:19:28 pm »

Yes, you are describing a distributed antenna system, and correctly understanding how to use a combiner in that application.

Shure shows how to engineer this scenario here https://www.shure.com/en-US/support/find-an-answer/antenna-distribution-2-rooms-4-antennas

Of the many factors to consider when designing such a system, the most important is to avoid conditions where a single carrier, transmitted from a desired performance location, arrives at the receiver through multiple paths that sum to a RSS that is low enough to cause a dropout or audible noise.  Since microphone TX are mobile devices and not rigidly fixed in spatial relation to their RX antennas, each path will impart a unique and dynamic loss or gain and phase shift on the signal.  Combiners and splitters are components that can cause phase shift, as are amplifiers and the electrical length of the entire path.

One of the easiest and most inexpensive ways to mitigate a destructive summing of zones is to insert variable attenuation or amplification into each path, which the user can adjust during or after system deployment.  Variable phase shifting would be wonderful, but I suspect way too expensive.  It's also worth noting that receiver diversity is an enormous factor in DAS, since by design it makes momentary phase cancellations on one side of the diversity pair inaudible.

So, sorry for beating the question to death here.. The way I described the application with the Sennheiser 212 unit is a valid approach. BUT, (this was my original question): phase issues etc can be an issue. (I asked originally if it's possible one can see a 180deg phase shift from one antenna comparing to the other by using different lengths and gauge coax cables) so it would null out at the receiver end.

EDIT: I read the link you provided, and that's also been my understanding of it the whole time. But in my example antennas would overlap each other, so now I understand that overlapping antennas can deal more damage, obviously. Thumbs up, thanks.

But. Can I ask. Let's say you have two whips in free field, spaced just a few meters apart. Will it be a very noticable reduction in performance comparing to just using one whip (or paddle for example)? Can you end up with detrimental performance? I know how it works with wavelengths comparing to spacing between antennas. But in a "random" situation, will the performance of the WL straight up suck, or is it more that some planets needs to align in a line first, before one one notices it once in a while?
« Last Edit: September 16, 2019, 01:32:45 pm by Miguel Dahl »
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Jason Glass

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Re: Simple 2-way radio mic antenna solution
« Reply #18 on: September 16, 2019, 03:15:27 pm »

Let's say you have two whips in free field, spaced just a few meters apart. Will it be a very noticable reduction in performance comparing to just using one whip (or paddle for example)? Can you end up with detrimental performance? I know how it works with wavelengths comparing to spacing between antennas. But in a "random" situation, will the performance of the WL straight up suck, or is it more that some planets needs to align in a line first, before one one notices it once in a while?

It will range from being less than ideal to straight up sucking.

For simplicity, we'll assume free space, theoretical lossless cables of equal length, and antennas of exactly equal gain.  We'll also assume a theoretical combiner that has perfect phase coherence from input ports to output port and excellent port isolation.

Every passive two-way combiner imparts a mandatory -3 dB loss, at best, and additional insertion losses from real-world imperfections.

At all positions where the TX antenna is located an equal distance to both RX antennas, the signal will travel through two paths and arrive at the combiner in-phase and constructively interfere, and sum to +3 dB gain, merely making up for the -3 dB combiner loss.  FWIW, all of these positions are located within a plane passing halfway between the antennas and orthogonal to the axis formed by a line between the two antennas.

At every other position where the difference in distance is a multiple of 1 wavelength, constructive interference will occur but not enough to make up for the combiner loss.  The determining factor here is the difference in signal strength between the two paths.  The closer together their signal strengths are, in perfectly aligned phase, the less the net loss will be.

At all positions where the difference in distance is 1/2 wavelength, the signal will arrive 180 out of phase and destructively interfere and sum to -∞ dB loss.

At every remaining position where the difference in distance is a multiple of 1/2 wavelength, destructive interference will occur in addition to combiner loss.  The determining factor here is the difference in signal strength between the two paths.  These interference losses will be severe because the antennas aren't located very far apart, assuring that the difference in signal strength between the two paths is small.  The closer together their signal strengths are, at 180 out of phase, the greater the net loss will be.

Every position in between all of those described above also results in a phase shift and net loss that is worse than a single antenna without a combiner.

This scenario is an intentional creation of multiple signal paths, resulting in textbook multipath interference and net losses for all but a relatively small volume of TX locations.

Here is a fabulous simulator to visualize what we're discussing with animated graphics.  https://phet.colorado.edu/sims/html/wave-interference/latest/wave-interference_en.html Here's a quick video primer on using it to show multipath interference.  http://cleanwirelessaudio.com/PHET_Multipath_Simulation.mp4

It's a really good thing that our gigs aren't in theoretical free space!  Reflections are enormously helpful to real-life RF work.  But it's also good that we can use free space as a modeling tool to optimize our systems because reflections are staggerlingly complex to calculate in 3D space.  Murphy's law applies.  Our best chance at success is to remove as many of his possibilities as we are allowed and free space lets us at least anticipate the basics.
« Last Edit: September 17, 2019, 12:01:06 pm by Jason Glass »
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Re: Simple 2-way radio mic antenna solution
« Reply #18 on: September 16, 2019, 03:15:27 pm »


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